1. Field of the Invention
The present invention relates to a method and apparatus for digitally transferring audio data and audio-related information.
2. Description of the Related Art
As used herein, “audio data” is distinguished from “audio-related information”. The “audio data”is the information representing the actual sounds to be reproduced. The “audio-related information”is defined as any information that does not directly represent the actual sounds to be reproduced, e.g., category codes, source numbers, and channel numbers.
Conventional methods for digitally transferring audio data and audio-related information are based on, for example, international standards such as IEC60958 and IEC61937.
IEC60958, which is used for transferring 2-channel linear PCM data, is used in a wide range of digital data transfer applications including CDs and DVDs. IEC61937 is used for the transfer of data other than linear PCM data, such as compressed data (e.g., data compressed according to the MPEG standards). In recent years, IEC61937 is used in applications where the multi-channel audio outputs from a DVD apparatus are utilized for reproduction by an external decoder amplifier connected to the DVD apparatus.
The aforementioned conventional methods for transferring audio data and audio-related information aim at simply reproducing the transferred audio data.
In recent years, frequencies such as 96 kHz, up to 192 kHz, are adopted as sampling frequencies for DVDs and the like.
However, the aforementioned conventional transfer standards do not support sampling frequencies such as 96 kHz and 192 kHz. Therefore, when a player is connected to an external apparatus, the original audio data is subjected to a down sampling process or the like for converting the sampling frequency to 48 kHz before transfer. In this case, however, there is a problem in that the external apparatus receiving the transferred data is not aware of the value of the sampling frequency at which the original audio data was sampled.
FIG. 5 shows an audio data transfer apparatus 100 as a consumer-use digital audio apparatus. The audio data transfer apparatus 100 includes an audio data transmitter 101 for transmitting audio data and an audio data receiver 102 for receiving the transmitted audio data, the transmitter 101 and the receiver 102 being interconnected via an audio data transfer interface 103 designed for audio data transfer.
Tables 1 and 2 show transfer conditions for transferring audio data from the audio data transmitter 101 in the aforementioned audio data transfer apparatus 100.
TABLE 1Signal processingSampling frequencyMonitoringor data transferof transferredfrequency atprocessingdatathe receiving enddown-samplingsampling frequencysampling frequencyafter down-after down-samplingsamplingup-samplingsampling frequencysampling frequencyafter up-samplingafter up-samplingsamplingsampling frequencysampling frequencyconversionafter samplingafter samplingconversionconversionN times speedoriginal samplingN timestransferfrequency 1/N times speedoriginal sampling 1/N timestransferfrequency
TABLE 2Signal processing ordata transfer processingMonitoring abilitydown-samplingpossibleup-samplingpossiblesampling conversionpossibleN times speed transferimpossible 1/N times speed transferimpossible
As shown in Table 1, the transfer conditions for the audio data transfer apparatus 100 include up-sampling, down-sampling, sampling conversion, N times speed transfer, 1/N times speed transfer, etc., in addition to transferring the original audio data at the originally-intended speed.
Under up-sampling, down-sampling, sampling conversion conditions, the sampling frequency of the transferred audio data differs from that of the original audio data. However, the audio-related information which is transferred in the conventional audio data transfer apparatus 100 does not include information concerning the sampling frequency of the original audio data. As a result, there is a problem in that the audio data receiver 102 cannot obtain any information concerning the sampling frequency of the original audio data.
Under conditions where audio data is transferred at an N times transfer speed or a 1/N times transfer speed, the sampling frequency of the original audio data is retained; however, the audio data receiver 102 cannot monitor the transferred data as it is (see Table 2). In order to properly monitor the audio data transferred under N times transfer or 1/N times transfer conditions, it is necessary to reproduce the audio data at an N times or 1/N times speed, or to reproduce the audio data at the sampling frequency of the original audio data while storing the transferred audio data in large memories. In the case of non-PCM data, subjecting the transferred data to a decoding process which does not involve some special measures would result in the audio information being reproduced with unwanted interruptions because the audio data receiver 102 often cannot perform the decoding process in time.
FIG. 6 illustrates an example of a conventional audio data/audio-related information recording format 300. As shown in FIG. 6, the conventional audio data/audio-related information recording format 300 contains no information indicating the sampling frequency of the original audio data or no information indicating the transfer speed of the transferred audio data. Hence, it is impossible for the audio data receiver 102 to detect the sampling frequency of the original audio data or the transfer speed of the transferred audio data.